1. Field of the Invention
The present invention relates to an apparatus for reducing the vibrations of periscopes and other vessel devices which extend into a liquid medium when in use. More specifically, the present invention relates to an apparatus for reducing the vibrations of submarine's periscopes and other extended-in-use vessel devices, by means of a damping arrangement, wherein the vibrations are induced by the relative movement of the submarine or vessel with respect to the liquid medium.
2. Description of the Prior Art
When periscopes or other similar vessel devices, which are extended in use, are moved through water, the devices may have vibrations set up therein which impair that use. These vibrations may also render the use of the devices impossible. One cause of these vibrations has been determined to result from the separation of vortices. Under certain conditions, a Karman vortex path can form. The formation and magnitude of vortices depend upon parameters which may include the diameter of the periscope or tubular device, the approach velocity and the temperature-dependent viscosity of the water. Other factors influencing the formation of vortices include other contemporaneously extended apparatuses and turbulence produced by a submarine's other devices which extend into the water including, for example, turbulence produced by the submarine's conning tower. With increasing velocity of the periscope or other extended-in-use device with respect to the water, the frequency of the vortex will increase. As the natural frequencies, including the fundamental frequency, are approached, resonant vibrations will occur which may impair the functionality of the device. The resonant vibrations will persist as the relative velocity increases beyond that corresponding to the natural frequency. Thus, in the case of a periscope, its functionality will remain impaired over a wide velocity range for which its utilization is required.
Vibrations caused by vortices can be classified according to three general categories, as follows: (1) deflections parallel to the direction of travel; (2) lateral deflections perpendicular to the direction of travel; and (3) deflections intermediate between the directions of parallel to and perpendicular to travel. Lateral vibrations perpendicular to the direction of travel will generally be of a greater magnitude than vibrations parallel to the direction of travel, since the vortices alternately dissipate and reform on each side of the extended-in-use device. For example, for a periscope having an extended free length of about 4.2 meters and traveling at a certain speed, lateral deflections of .+-.40 mm were observed. The observed lateral deflections correspond to an acceleration of 10 g. Deflections parallel to the direction of travel of .+-.8 mm were observed. The frequency of the observed vibrations was between 7 and 8 Hz. Vibrations of the periscope, as just described, have a detrimental effect on the periscope optical system whereby accurate observations cannot be attained.
Proposals have already been made for the damping of tall, slender structures, such as smoke stacks or masts, where the vibration is due to wind. Such proposals include German Patent Publications DE-AS No. 28 06 757 and DE-PS No. 32 14 181, which disclose the installation of annular damping weights by means of spring elements or vibration-damping elements close to the top ends of the structures. These proposals are not transferable to periscopes or similar devices on submarines for which the vibrations are produced by the streaming of water and for which special conditions pertaining to submarines must be observed.
A proposal has also previously been made to reduce the vibrations of extendible antenna supports for submarines. In particular, German Patent Publication DE-AS No. 23 17 840 discloses a proposal providing a tubular antenna support with a similarly extendible, streamlined cover. An undesirable aspect of this proposal is that the cover impairs the provision of a rigid seating for the portion of the antenna extending above the tower. Therefore, in this example of background art, an additional provision was made to attach the antenna carrier upper support via a traverse or cross-tie rod to a fixed guide, wherein the traverse is located inside the extendible cover. The traverse moves along the fixed guide as the antenna is extended. Although the streamlined cover can exert a favorable influence on the vortex formation, it cannot prevent the occurrence of vibrations for all conditions encountered in the operation of a submarine. The drawbacks associated with this proposal are the high costs for the extendible cover, the traverse rod and the fixed guide assembly, the low stiffness of the upper support near the traverse, and the elevation of the center of gravity of the submarine resulting from the additional elements.